The present invention generally relates to the field of pulsed laser control. More specifically, the present invention describes an external apparatus and method adaptable to most lasers for selecting a predetermined number of laser pulses.
During material processing with lasers it is often necessary to apply a single pulse or a low number of pulses to a spot of the device or material being treated. In principle, this could be done by firing the laser from an external trigger when a pulse is required (single pulse operation). However, most lasers, especially high power lasers, are designed such that the thermal focal power of the gain medium (i.e., the laser rod or slab) is part of the laser resonator. When the laser is turned on it takes several shots for this focal power to develop and for the resonator to function properly. During these first few pulses the output energy and spatial profile of the laser beam are unpredictable. Obviously, the same problem arises when the laser is run in single pulse operation. Hence, single pulse operation is not an option when, as in many critical applications, well characterized laser pulses are required. In addition, to generate the current pulses for the lamp(s), conventional laser power supplies typically use a xe2x80x9cpulse-forming networkxe2x80x9d (PFN) that is charged from a reservoir capacitor bank with a technique called xe2x80x9cresonance charging.xe2x80x9d This technique also requires a few pulses before the PFN produces reproducible current pulses. To overcome these problems, it is customary to have the laser running continuously in thermal and electrical steady-state, and to have the laser beam physically blocked. In the prior art, blocking of the laser beam is typically achieved inside the resonator for those lasers capable of limiting the number of pulses. It is advantageous to block the beam external to the resonator. With external blocking the energy is extracted from the gain medium to avoid uncontrolled parasitic oscillation. Further, lasers without the ability to limit the number of pulses require an external method of blocking the beam if they are to be made capable of limiting the number of pulses emitted. Then, when one or a few pulses are required, the operator manually removes the block for an appropriate amount of time and then blocks the beam again after the desired number of pulses has passed. This approach becomes less and less feasible as the pulse repetition rate of the laser increases. The repetition rate of the laser pulses at some point becomes too rapid for the operator to physically move a block and, at all repetition rates; it is possible that a partial pulse could be allowed to pass.
The present invention discloses an apparatus and method that provides for the application of a predetermined number of lasers pulses in situations where manual operation is prohibited by physical limitations imposed by high repetition rate lasers. The approach, however, is not limited to high repetition rate lasers nor high power lasers. The approach can be applied to low repetition rate lasers and low power lasers as well. The disclosed invention can be fabricated as an external assembly to the laser and, therefore, adapted to lasers not having the ability to limit the pulses to a predetermined number.
The present invention is divided into three major blocks. In a first operation the initiation of the selection of pulses is synchronized with the operation of the laser. That is, initiation of the sequence for application of the laser pulse(s) is delayed until the advent of the next laser pulse. In this manner the timing of the second operation is aligned with the repetition rate of the laser.
The second operation begins with the occurrence of the first laser pulse after the initiation. In this operation, a delay generator allows laser pulses to pass for a duration of time predetermined by the number of laser pulses desired.
The third operation is the placement of a physical shutter that blocks or allows the laser pulses depending upon the status of the delay generator output of operation two.